The 7900GT costs the same as the 7800GTX, and is in fact the same card, just built with smaller transistors (90nm I think). The x1900xt will give you a better 3DMark score I believe, but for general gaming, if you really wanted to buy a new card, get the 7900GT.
I would stick with the 7800GT though, its more than good enough to play most games today farely smoothly! It could be that you just had a lot of processes running in the background when you ran 3DMark05, cause I scored like 50 more after just a reboot, so try clearing some of those unescessary processes out!
AMD64 X2 4400+
DFI Lanparty Ultra-D
2GB Kingston Hyper-X (2-3-2-6)
TT Big Typhoon
eVGA 7900GT KO Superclocked (650/1650)
250GB Maxtor DiamondMax SATA2 HDD
Hiper Type-R 580W SLi PSU
19" Sony Trinitron CRT